The embodiments described herein relate to a magnetic resonance imaging apparatus (MRI) and an image generating method, and particularly to a magnetic resonance imaging apparatus which executes scans for transmitting RF pulses to a subject within a static magnetic field space and collecting or acquiring magnetic resonance signals from the subject thereby to generate an image of the subject, and an image generating method for detecting the position of each region in a subject, which is obtained by executing each scan and thereby generating an image.
A magnetic resonance imaging apparatus executes scans for applying an electromagnetic wave to a subject lying within a static magnetic field space thereby to excite spins of proton in the subject by a nuclear magnetic resonance phenomenon and acquiring magnetic resonance signals generated by the excited spins. This is of an apparatus that generates a slice image with respect to a tomographic plane of the subject, based on the magnetic resonance signals obtained by the scans.
There is a case in which body-motion artifacts occur in the generated slice image where body motion occurs in the subject upon imaging the subject using the magnetic resonance imaging apparatus. When, for example, the heart or abdominal region of the subject is imaged or photographed, body motion artifacts occur due to body motion such as breathing exercises, cardiac motion or the like, thus degrading the quality of the image.
Thus, there have been proposed methods for solving the problem of the degradation in the image due to the body motion artifacts. One method thereof is that upon imaging or photography under normal respiration, for example, an excitation section of a subject is corrected in real time according to a change in the position of a diaphragm and each magnetic resonance signal is always measured from the same section, thereby preventing the degradation in the image due to the body motion artifacts. An imaging sequence is changed or imaging data is selected through the use of acquired navigator echoes, thereby preventing degradation in image quality due to body motion artifacts (refer to, for example, Japanese Unexamined Patent Publication No. 2007-111188 and Japanese Unexamined Patent Publication No. 2007-98026).
As an approach for detecting the position of the diaphragm, which is used in these techniques, there has been known a method for tracking or scanning the motion of the diaphragm using navigator echoes and performing respiratory synchronization and gating using acquired navigator data (refer to, for example, Japanese Unexamined Patent Publication No. Hei 10(1998)-277010).
For example, a signal intensity profile obtained by plotting the relationship between a signal intensity I of navigator data and the position X in a navigator area is generated. In the generated signal intensity profile, for example, the position of the diaphragm lying in the boundary between the liver and lung is calculated and respiratory synchronization is performed.
As a method for calculating the position of the diaphragm in the signal intensity profile, there has been known, for example, a differential method or a Du method or the like (refer to, for example, JOURNAL OF CARDIOVASCULAR MAGNETIC RESONANCE, Vol. 6, No. 2, pp. 483-490, 2004).
However, as a result that as shown in a coronal image of FIG. 15, an imaging area IA for executing an imaging scan to acquire imaging data has overlapped with a navigator area NA corresponding to the position of acquisition of navigator data, signal disturbance due to slice interference occurs in the acquired navigator data. As indicated by a broken-line area of FIG. 16, a low-signal region occurs in a signal intensity profile. Here, the broken-line area shown in FIG. 16 indicates a signal intensity profile corresponding to a portion where the imaging area IA and the navigator area NA shown in FIG. 15 overlap. In doing so, it became difficult to obtain a stable analytic result by the conventional navigator data analyzing method shown above.
Thus, there has been considered a method for suppressing the occurrence of signal disturbance due to the interference of an imaging scan by using phase information of navigator data.
However, though the occurrence of the signal disturbance due to the interference of the imaging scan can be suppressed by using the phase information of the navigator data, a problem arises in that since variations are apt to occur in the phase at a region low in signal intensity, it is difficult to obtain the result of analysis of navigator data stably, thus causing degradation in image quality.
It is desirable that the problem described previously is solved.